[asphaltgambler]

[QUOTE .......any idea how many were made? I do see these come up for sale periodically on the various H-D forums. Big, big bucks...

So, which would ultimately make more power, Thunderheads or XR1000 heads? I remember when the old XL List guys tried to regain the land speed record for stock 1000 cc pushrod twins from some guys who set it with a Guzzi. They tried and tried at Maxton (and possibly Bonneville) and finally gave up on any kind of Ironhead. They could not use the Thunderheads in their class. They then went to an Evo based bike and quite easily regained the record. That tells me even the twin carbureted XR1000 heads won't make Evo levels of power.

Unless, of course, one removes the aircleaner, like Speeder...[/QUOTE]


Jeff - I don't know actual numbers - but from the few people that I consider credible sources of info on topics like this - maybe a 800-1000 sets. While the XR heads were much improved combustion chamber / valve angles / port shape that design is still dated even when looking at mid-80's technology.

For what it's worth - my heads are stock 1957 castings with XR / Baisley valves, red shift springs, Baisley roller tip rockers, port / flowed and duel plugged by Zippers. The piston dome shape makes up for the half grape-fruit size combustion chamber. But the port size and cam lobe limitation is really too small to support my engine configuration. Unfortunately, they really the only choice, what works best. The XR and Thunder heads have smaller ports and less material to work with so they are out.

Interestingly enough, Zippers performance campaigned a 96" pro-stock iron head Sportster in the mid-late 80's that not only was competitive but consistently won and set records and was feared when all rivals were running (new) EVO setups.

Of course the head castings were stock but had machined huge ports and relocated valves where an tube insert was pressed in and then brazed.